"ansible/setup.yml" is failing for "focal" distro with error 'ansible_distribution' is undefined '

**Environment details:**

- "arm64-graviton2" platform for Travis-CI
- Ubuntu 20.04(focal)

**Steps to reproduce the issue:**
Adding "arm64-graviton2" jobs for travis-CI. Please find the link below:
https://github.com/odidev/openwhisk/blob/master/.travis.yml

**Actual results and outputs:**
Getting following error:
```
The conditional check 'nginx.ssl.cert == "openwhisk-server-cert.pem"'
failed. The error was: error while evaluating conditional (nginx.ssl.cert ==
"openwhisk-server-cert.pem"): 'ansible_distribution' is undefined The error
appears to have been in '/home/travis/build/odidev/openwhisk/ansible/setup.yml':
line 61, column 5, but may be elsewhere in the file depending on the exact
syntax problem. The offending line appears to be: # Generate nginx
certificates - name: gen untrusted server certificate for host ^ here

```
Please find the travis link below:
https://travis-ci.com/github/odidev/openwhisk/jobs/440788935

We've added "arm64" platform for travis-CI as well but "./tools/travis/docker.sh" script was failing there due to "Docker Application Container Engine" was failing to start. Hence we tried for "arm64-graviton2" platform for Travis-CI, and docker issue got resolved in that but here we are facing above issue. We're getting same error for "focal" distribution of both the platforms(Amd64 & Arm64).
Are you planning on updating distro from xenial to focal? If interested I can add and raise a PR. But as a start can I get suggestion on above issue?

Start with .travis.yml and reproduce the focal jobs, then inspect ansible/setup.yml around line 61 and tools/travis/docker.sh for the platform setup. Compare the focal and xenial paths to determine why ansible_distribution is undefined. Done means the focal jobs complete successfully on the affected platforms without the certificate conditional error.

Written by the indexing model from the issue text.